Pyrido[2,3-d]pyrimidines are a class of compounds known to have a variety of biological activities. U.S. Pat. No. 3,288,792 discloses certain 2,4-diamino-6-alkyl and phenylalkyl pyrido[2,3-d]pyrimidines which are said to have antibacterial activity. U.S. Pat. No. 4,512,992 describes 2,4-diamino-6-(dialkoxybenzyl)-5-methylpyrido[2,3-d]pyrimidine as a treatment for psoriasis. Considerable interest recently has focused on a series of anticancer agents which are 2,4-diamino- and 2-amino-4-hydroxy derivatives of pyrido[2,3-d]pyrimidines having a 6-alkylbenzoyl-L-glutamic acid moiety; U.S. Pat. Nos. 4,684,653, 4,902,796, and 4,871,746. Of particular interest among this latter series is the compound lometrexol, commonly referred to as 5,10-dideazatetrahydrofolic acid or DDATHF, which has been shown to have clinical efficacy against solid tumors; Proc. Amer. Assoc. Cancer Res., 31, 1053 (1990). An object of this invention is to provide non-glutamate derivatives of DDATHF having antineoplastic activity.